Vascular smooth muscle cell (SMC) proliferation is a central component of the pathophysiologic response to injury. Hepatoma derived growth factor (HDGF) is a potent SMC mitogen that is not normally present in the rat carotid, but becomes highly expressed in a nuclear pattern in the neointima after balloon injury. HDGF is also present in human vascular disease being co-expressed with PCNA in proliferating SMC in human atherosclerotic plaques, but the mechanisms by which HDGF produces SMC proliferation are unknown. This is due in part to the considerable controversy regarding the nuclear function of growth factors, in general and in the vascular wall in particular. Like HDGF, many growth factors in the vascular wall, including platelet derived growth factor (PDGF) and fibroblast growth factor (FGF), are nuclear targeted. However the nuclear function of growth factors, if any, has not been demonstrated. The overall goal of this grant proposal is to elucidate the nuclear mechanism(s) by which HDGF stimulates SMC proliferation in vitro and in response to injury. In our pilot studies, we found that single point mutations in a putative bipartite nuclear localization sequence block HDGF nuclear targeting and mitogenesis. We also now have evidence from a yeast two hybrid screen and co-immunoprecipitation (IP) studies that HDGF binds the key nuclear cell cycle regulatory protein p55cdc. Based upon these studies, we propose to determine the mechanisms regulating HDGF nuclear entry; whether nuclear entry is necessary for mitogenic function; and whether the nuclear function is regulation of the cell cycle control protein p55cdc. The significance of the proposed studies is that by linking HDGF nuclear entry to regulation of the cell cycle control protein p55cdc, a critical step in the nuclear action of HDGF is revealed and brings together for the first time the nuclear mechanism(s) of growth factor action in SMC. Indeed, understanding how HDGF regulates cell proliferation may provide additional insights into how growth factors regulate cellular growth in general.